The present invention relates to a system and method for non-invasive imaging of retinal function.
In some degenerative retinal diseases, retinal function may be altered even before changes in the retinal tissue occur which are detectable by current, non-functional, diagnostic imaging techniques. In glaucoma, for example, up to 50% of the retinal axons may degenerate before symptoms of pathology or reduced visual acuity are noticed, even by the patient. Due in part to limitations in current diagnostic technology, it is estimated that more than 3,000,000 patients in the U.S. alone are currently undiagnosed victims of treatable eyesight impairment.
Common current ophthalmic diagnostic imaging techniques such as retinal photography, fluorescence angiography, as described, e.g. in WO 92/03086. and retinal tomography, do not directly reveal the state of the retina""s function. Retinal photography detects abnormalities in the static coloration of the retina, caused, for example, by hemorrhages, hard yellow exudates, xe2x80x9csilverxe2x80x9d or xe2x80x9ccopperxe2x80x9d blood vessel opacification, xe2x80x9ccotton woolxe2x80x9d spots, and blanching of the retina due to retinal edema or anoxia. These correlate with functional deficits, but do not reveal the functional integrity of the retina per se, and may only become obvious some time after pathology has established itself. Similarly, fluorescence angiography, which relies on the pattern of fluorescent labeling of the retina""s microvascular network by injected probes such as fluorescein and indocyanine green, provides only indirect information about retinal function. What it does directly reveal are flaws in the retinal vasculature, correlated with, but often subsequent to, certain functional deficits. Finally, retinal tomography can reveal anatomical changes due to reduced numbers of retinal axons, but again, this technique is insensitive to the functional integrity of the retina itself.
The present invention, however, may be used to directly measure the functional integrity of the retina. It is intended to detect electrical activity-dependent changes in the optical properties of the retina, either non-invasively, or after retinal labeling by a specially designed fluorescent probe. These activity-dependent changes are directly related to local retinal responsiveness to visible light. The present invention measures retinal function in one of two modes: an intrinsic signal mode, and a fluorescence signal mode.
The intrinsic signal mode relies on any one of a number of functional optical signals resulting from the retina""s native properties. Upon illumination, the functioning retina undergoes changes in its reflectance which are due to alterations in many physiological parameters, including, but not limited to, blood flow, blood oxygenation, light scattering from the retinal axons, light scattering and/or pigment bleaching from the retinal pigmented epithelium, and rod or cone photopigment bleaching. The retina itself thus provides a rich source of optically measurable functional signals, occurring at many different wavelengths and with many different time courses.
When the method according to the present invention is operated in the fluorescence signal mode, the retina of interest may first be labeled with a fluorescent voltage-sensitive probe. Such a probe is a compound which binds to the membranes of retinal neurons and alters its fluorescent properties (emitting more or less light, or shifting its spectral properties, under constant illumination), depending on the electrical field across the membrane of a neuron within which it is bound. Because transmembrane electrical fields are altered when neurons are active, light measurements using such a fluorescent voltage-sensitive probe provide a means for directly assaying retinal activity. Alternatively, a fluorescent signal may be measured by choosing excitation and emission wavelengths appropriate to an intrinsic fluorescent chromophor, such as NADH.
The magnitude of either type of functional optical signal is influenced by the optical properties of the resting retina. Light must pass through any unusually opaque or light-scattering tissue which interposes itself between the active retinal tissue and the illuminator or detector. This modulates the functional signal in a way which may also reveal static abnormalities in the retina""s optical properties.
The differences between the reflectance or fluorescence images taken from the resting retina and from the active retina may be encoded as a differential image, in which differences in brightness are due entirely to functional activity, as filtered though any optically manifested retinal abnormalities. Both types of information are of clinical relevance.
The functional changes in reflectance/fluorescence which the present invention measures may be quite small, often no more than 0.1% of the total light received by the imaging device. Small changes in reflectance are not obvious when they occur against a high background level of reflected or fluorescent light, and therefore functional optical imaging has not been previously used to reveal ophthalmic diagnostic information.
A device designed to use functional changes in retinal reflectance (measured point by point, not in image format) to reveal the retina""s health is disclosed in U.S. Pat. No. 5,485,229. This prior art invention exclusively uses the previously observed phenomenon of light-scattering changes which occur in active axons, in this case, in the retinal nerve fiber layer. The present invention, however, differs from and improves on this prior art device in several ways. One important improvement over the prior art is in the adaptability of this invention to recording and evaluating multiple types of functional signals, at multiple wavelengths, in opposition to the prior art, which is restricted to one signal type and imaging wavelength. A second point of difference is that the present invention simultaneously images hundreds of thousands to millions of retinal points, improving on the single point resolution of the prior art. A third key difference is in the present invention""s application of differential image analysis to a functional retinal signal.
The differential image analysis for revealing a functional signal according to the present invention is novel for use in clinical imaging of the retina, in that it selectively emphasizes metabolically induced variations in reflectance or probe fluorescence, over time, across the retinal surface by removing the stable portion of the image from consideration. A prior art method which relies on the temporal pattern of blood-borne inflow of contrast agents to the retina is described in U.S. Pat. No. 5,150,292, which shares some aspects of the image analysis technique outlined here. However, the method of said patent cannot reveal functional retinal signals, as the present method is especially designed to do.
In accordance with the present invention, there is therefore provided a system for imaging reflectance changes, intrinsic or extrinsic fluorescence changes of a retina due to normal retinal function, comprising an imaging illuminator for illumination of the retina; a retina-stimulating illuminator for inducing an optically detectable functional response signal; an imaging device receiving light from the retina via retinal imaging optics; image acquisition means for digitizing and storing images received from said imaging device, and computer means for controlling the operation of the system and for processing said stored images to reveal a differential optically detectable functional response signal corresponding to the retina""s function.
The invention further provides a method for imaging reflectance changes, intrinsic or extrinsic fluorescence changes of a retina due to normal retinal function, comprising the steps of illuminating a retina with light from a stable light source; imaging and recording the retina""s image as illuminated to form a baseline image; illuminating the retina with light having wavelengths appropriate for inducing an optically detectable functional response signal; imaging and recording the retina""s image after or during the induction of said functional response signal; illuminating the retina with light having wavelengths appropriate for measuring said signal; performing a differential analysis of said baseline and response images, whereby a differential optically detectable functional response signal corresponding to the retina""s normal function is revealed.